Anatomy accommodating prosthetic intervertebral disc with lower height

ABSTRACT

An intervertebral disc includes a superior endplate having an upper vertebral contacting surface and a lower bearing surface, wherein the upper vertebral contacting surface of the superior endplate has a central portion that is raised relative to a peripheral portion of the superior endplate, and wherein the lower bearing surface has a concavity disposed opposite the raised central portion. The disc includes an inferior endplate having a lower vertebral contacting surface and an upper surface, wherein the lower vertebral contacting surface of the inferior endplate has a central portion and wherein the upper bearing surface has a concavity disposed opposite the central portion. A core is positioned between the upper and inferior endplates, the core having upper and lower core bearing surfaces configured to mate with the bearing surfaces of the upper and inferior endplates. The upper vertebral contacting surface of the superior endplate has a different shape than the lower vertebral contacting surface of the inferior endplate.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 15/230,116 (Attorney Docket No. 29850-729.302), filed Aug. 5,2016, which is a continuation of U.S. patent application Ser. No.14/339,280 (Attorney Docket No. 29850-729.301), filed Jul. 23, 2014, nowU.S. Pat. No. 9,421,107, which is a continuation of U.S. patentapplication Ser. No. 13/647,933 (Attorney Docket No. 29850-729.201),filed Oct. 9, 2012, now U.S. Pat. No. 8,808,384, which claims thebenefit of U.S. Provisional Application No. 61/546,848 (Attorney DocketNo. 29850-729.101), filed Oct. 13, 2011, the entire contents of whichare incorporated herein by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to medical devices and methods. Morespecifically, the invention relates to intervertebral prosthetic discsand methods of preserving motion upon removal of an intervertebral disc.

Back pain takes an enormous toll on the health and productivity ofpeople around the world. According to the American Academy of OrthopedicSurgeons, approximately 80 percent of Americans will experience backpain at some time in their life. In the year 2000, approximately 26million visits were made to physicians' offices due to back problems inthe United States. On any one day, it is estimated that 5% of theworking population in America is disabled by back pain.

One common cause of back pain is injury, degeneration and/or dysfunctionof one or more intervertebral discs. Intervertebral discs are the softtissue structures located between each of the thirty-three vertebralbones that make up the vertebral (spinal) column. Essentially, the discsallow the vertebrae to move relative to one another. The vertebralcolumn and discs are vital anatomical structures, in that they form acentral axis that supports the head and torso, allow for movement of theback, and protect the spinal cord, which passes through the vertebrae inproximity to the discs.

Discs often become damaged due to wear and tear or acute injury. Forexample, discs may bulge (herniate), tear, rupture, degenerate or thelike. A bulging disc may press against the spinal cord or a nerveexiting the spinal cord, causing “radicular” pain (pain in one or moreextremities caused by impingement of a nerve root). Degeneration orother damage to a disc may cause a loss of “disc height,” meaning thatthe natural space between two vertebrae decreases. Decreased disc heightmay cause a disc to bulge, facet loads to increase, two vertebrae to rubtogether in an unnatural way and/or increased pressure on certain partsof the vertebrae and/or nerve roots, thus causing pain. In general,chronic and acute damage to intervertebral discs is a common source ofback related pain and loss of mobility.

When one or more damaged intervertebral disc cause a patient pain anddiscomfort, surgery is often required. Traditionally, surgicalprocedures for treating intervertebral discs have involved discectomy(partial or total removal of a disc), with or without interbody fusionof the two vertebrae adjacent to the disc. When the disc is partially orcompletely removed, it is necessary to replace the excised disc materialwith natural bone or artificial support structures to prevent directcontact between hard bony surfaces of adjacent vertebrae. Oftentimes,pins, rods, screws, cages and/or the like are inserted between thevertebrae to act as support structures to hold the vertebrae and anygraft material in place while the bones permanently fuse together.

A more recent alternative to traditional fusion is total discreplacement or TDR. TDR provides the ability to treat disc related painwithout fusion provided by bridging bone, by using a movable,implantable, artificial intervertebral disc (or “disc prosthesis”)between two vertebrae. A number of different artificial intervertebraldiscs are currently being developed. For example, U.S. Pat. Nos.7,442,211; 7,531,001 and 7,753,956 and U.S. Patent ApplicationPublication Nos. 2007/0282449; 2009/0234458; 2009/0276051; 2010/0016972and 2010/0016973 which are hereby incorporated by reference in theirentirety, describe artificial intervertebral discs with mobile bearingdesigns. Other examples of intervertebral disc prostheses are theCharité® disc (provided by DePuy Spine, Inc.) MOBIDISC® (provided by LDRMedical (www.ldrmedical.fr)), the BRYAN Cervical Disc (provided byMedtronic Sofamor Danek, Inc.), the PRODISC® or PRODISC-C® (from SynthesStratec, Inc.), the PCM disc (provided by Cervitech, Inc.), and theMAVERICK® disc (provided by Medtronic Sofomor Danek).

These known artificial intervertebral discs generally include superiorand inferior endplates which locate against and engage the adjacentvertebral bodies, and a core for providing motion between the plates.The core may be movable or fixed, metallic, ceramic or polymer andgenerally has at least one convex outer surface which mates with aconcave recess on one of the plates in a fixed core device or both ofthe plates for a movable core device. In order to implant theseintervertebral discs, the natural disc is removed and the vertebrae aredistracted or forced apart in order to fit the artificial disc in place.Depending on the size of the disc space, many of the known artificialdiscs have a height which is higher than desired resulting in anunnaturally over distracted condition upon implantation of the disc. Forexample, a smallest height disc available can be about 10-13 mm forlumbar discs and about 5-6 mm for cervical discs.

Currently available artificial intervertebral discs do not provide adesired low profile for some patients with smaller disc heights. Itwould be desirable to provide a lower height disc which mimics moreclosely the natural anatomy for smaller patients.

In addition, the vertebral body contacting surfaces of many of the knownartificial discs are flat. The inventor has recognized that this flatconfiguration does not generally match the surfaces of the vertebralbodies resulting in less than ideal bone to implant contact surfaces. Itwould be desirable to provide a more anatomically shaped vertebral bodycontacting surface for an artificial disc.

According to an aspect of the present invention, an intervertebral disccomprises: a superior endplate having an upper vertebra contactingsurface and a lower bearing surface, wherein the upper surface of theupper endplate has a domed central portion and wherein the lower bearingsurface has a concavity disposed opposite the domed central portion; aninferior endplate having a lower vertebra contacting surface and anupper surface, wherein the lower surface of the lower endplate has adomed central portion and wherein the upper bearing surface has aconcavity disposed opposite the domed central portion; a core positionedbetween the superior and inferior endplates, the core having upper andlower surfaces configured to mate with the bearing surfaces of thesuperior and inferior endplates; and wherein the domed central portionof the superior endplate has a height greater than a height of the domedcentral portion of the inferior endplate.

According to another aspect of the present invention, an intervertebraldisc comprises: a superior endplate having an upper vertebral contactingsurface and a lower bearing surface, wherein the upper vertebralcontacting surface of the superior endplate has a central portion thatis raised relative to a peripheral portion of the superior endplate, andwherein the lower bearing surface has a concavity disposed opposite theraised central portion; an inferior endplate having a lower vertebralcontacting surface and an upper surface, wherein the lower vertebralcontacting surface of the inferior endplate has a central portion thatis raised relative to a peripheral portion of the inferior endplate andwherein the upper bearing surface has a concavity disposed opposite thecentral portion; and a core positioned between the upper and inferiorendplates, the core having upper and lower core bearing surfacesconfigured to mate with the bearing surfaces of the upper and inferiorendplates; wherein the upper vertebral contacting surface of thesuperior endplate has a different shape than the lower vertebralcontacting surface of the inferior endplate.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention are well understoodby reading the following detailed description in conjunction with thedrawings in which like numerals indicate similar elements and in which:

FIG. 1A is a schematic cross-sectional view of a vertebra including anintervertebral disc according to an aspect of the present invention;

FIG. 1B is a schematic cross-sectional view of a vertebra including anintervertebral disc according to the prior art;

FIGS. 2A-2C are top perspective, side, and side, cross-sectional views,respectively, of an intervertebral disc according to an aspect of thepresent invention;

FIGS. 3A-3E are top, bottom, side, anterior cross-sectional, and sidecross-sectional views, respectively, of a superior endplate of anintervertebral disc according to an aspect of the present invention;

FIGS. 4A-4E are top, bottom, side, anterior cross-sectional, and sidecross-sectional views, respectively, of an inferior endplate of anintervertebral disc according to an aspect of the present invention;

FIG. 5 is a side cross-sectional view of an intervertebral discaccording to the prior art;

FIGS. 6A, 6B, and 6C illustrate several of various possible geometriesfor raised central portions of a superior endplate according to aspectsof the present invention;

FIG. 7 is a graph of the average radius of curvature of the superiorendplate for a collection of patients separated by disc level; and

FIG. 8 is a graph of the average depth of the superior endplateconcavity for a collection of patients separated by disc level.

DETAILED DESCRIPTION

FIGS. 2A-2C show an intervertebral disc 27 that comprises superior andinferior endplates 33 and 35 that, as seen in FIG. 1A, are sized andshaped to fit within an intervertebral space 37. Each endplate 33 and 35has a vertebral contacting surface 39 and 41, respectively, and an innersurface 43 and 45, respectively. As seen in the cross section of FIG.2C, the superior endplate 33 has a first or lower bearing surface 47 onthe inner or lower surface 43 of the superior endplate, and the inferiorendplate 35 has a second or upper bearing surface 49 on the inner orupper surface 45 of the inferior endplate. The bearing surfaces 47 and49 are spherical in shape, however other known bearing surface shapesmay also be used.

A mobile core 51 is configured to be received between the first andsecond bearing surfaces 47 and 49. The core 51 has at least one,ordinarily two, curved bearing surfaces 53 and 55. The core 51 isordinarily rigid, i.e., substantially inflexible and incompressible,however, the core may be flexible, compressible, and/or compliant withrigid bearing surfaces. In the embodiment shown in FIG. 2A-2C, the core51 is rigid and has first and second curved bearing surfaces 53 and 55for contacting the first and second curved bearing surfaces 47 and 49,respectively, of the superior and inferior endplates. The superior andinferior endplates 33 and 35 are articulable and rotatable relative toeach other via sliding motion of at least one of, ordinarily both of,the first and second bearing surfaces 47 and 49 over the core 51.

The inventor has recognized that providing raised or domed centralportions of both the upper vertebral contacting surface 39 and the lowervertebral contacting surface 41, the disc can be made smaller and closerin size to the natural disc providing more nearly natural motion. Theinventor has also recognized that the raised or domed central portionscan be advantageously raised in different amounts, to more accuratelymatch the anatomy of the majority of patients while further reducing theheight of the disc. Matching the natural disc height accurately leads tomore natural motion while overstuffing a large disc in a small discspace can lead to limited motion.

Discs which are too large for the size of a disc space of a patient arethought to be detrimental to the patient. A connection has been foundbetween fusion (no motion) at a disc level and increased degenerativedisc disease at adjacent disc levels. This leads to the conclusion thatoverly large discs with limited mobility may also result in increases indegenerative disc disease at adjacent levels. Placing a disc which istoo large for the disc space can also result in the unwanted result ofmigration sometimes leading to removal of the disc.

In one embodiment of an intervertebral disc system designed for use inthe cervical spine, a core has a diameter of about 8-10 mm, a height ofabout 4-6 mm, and a spherical radius of curvature of the bearingsurfaces of about 10-18 mm. The intervertebral discs 27 and 29 may beprovided in different heights to accommodate patient anatomy. Forexample, discs may be provided in multiple heights, such as of 4, 5, 6and 7 mm or in other size variations. The discs may also be provided indifferent sizes. In one example, for example different lengths in theanterior/posterior direction can be used to accommodate differentanatomies. Although different width discs can be used, in the preferredembodiment, the width of the discs is approximately the same for thedifferent disc sizes. The same core 51 and bearing surface configurationis preferably use in the discs of multiple sizes

In another embodiment of an intervertebral disc system designed for usein the lumbar spine, a core has a diameter of about 10-15 mm, a heightof about 5-10 mm, and a spherical radius of curvature of the bearingsurfaces of about 10-18 mm.

The superior endplate 33 (shown in FIGS. 3A-3E) and the inferiorendplate 35 (shown in FIGS. 4A-4E) each have left and right longitudinalsides 33 l and 33 r and 35 l and 35 r, when the disc is viewed from theanterior, and anterior and posterior ends 33 a and 33 p and 35 a and 35p.

At least the superior endplate 33 includes a projection 57 which engageswith a recess 59 of the core 51 to retain the core between the superiorand inferior endplates 33 and 35. The projection 57 can be in the formof an annular rim having a diameter D3 (FIG. 3E) and the recess 59 canbe in the form of an annular groove, usually having diameter equal to orslightly less than the diameter D3. In the embodiments shown in FIGS.1-3 and 6, the projection 57 extends 360° around the core 51. The coreand the projection are ordinarily shaped so that the part of the corebelow the recess can be passed through an opening defined by theprojection in a direction substantially along an axis of the openingdefined by the projection. In the embodiments illustrated, the endplate33 having the retaining ring or projection 57 has been illustrated asthe superior endplate. However, it should be understood that theinferior endplate 35 can be provided with a projection instead of or inaddition to having the projection on the superior endplate. Ordinarily,however, each endplate will not have a projection to keep thickness ofthe disc to a minimum and to increase range of motion of the endplatewithout a projection relative to the core and the endplates relative toeach other.

In addition to permitting sliding motion, the core 51 is movable withrespect to both of the superior and inferior endplates 33 and 35.“Movable” is specifically defined for purposes of describing themovability of the core 51 with respect to at least one of the superiorand inferior endplates 33 and 35 as meaning that the core is adapted tobe displaced in a direction toward or away from at least one of the leftand right longitudinal sides 33 l and 33 r and 35 l and 35 r andanterior and posterior ends 33 a and 33 p and 35 a and 35 p of at leastone of superior and inferior endplates. The core 51 is movable withrespect to both the superior and inferior endplates when theintervertebral disc system is in use implanted between the vertebrae ofa patient. However, the core 51 may have a non-movable position when thedisc is being implanted or before completion of a surgical implantationprocedure. In a preferred embodiment, the core 51 is movable withrespect to both the superior and inferior endplates in both theanterior-posterior direction and the left-right direction

Although the radius of curvature of the bearing surfaces of the core andthe bearing surfaces of the plates 47 and 49, as shown, aresubstantially the same (substantially congruent) for purposes ofdistribution of load and reduced wear, in other embodiments, at leastone of the first and second bearing surfaces 47 and 49, or the bearingsurfaces of the core can be modified with a groove, channel, depressionor flat on a portion of the bearing surface.

Instead of providing two curved bearing surfaces on the core andcorresponding curved bearing surfaces on the superior and inferiorendplates, one of the bearing surfaces may be another shape, such assubstantially flat (not shown). For example, the top bearing surface onthe core below the recess can be flat and the lower bearing surface onthe superior endplate above the projection can be flat. The firstbearing surface on the superior endplate can be larger than the firstbearing surface on the core to facilitate limited translational movementof the core relative to the superior endplate. If the second bearingsurface on the inferior endplate and the second bearing surface on thecore are curved, the superior and inferior endplates will be articulableand rotatable relative to each other via sliding motion. In anotherexample, the first bearing surface 47 on the inner surface 43 of thesuperior endplate and corresponding bearing surface 53 on the core 51can be flat while the upper bearing surface 49 on the upper surface 45of the inferior endplate 35 and the corresponding bearing surface 55 ofthe core can be cylindrically curved in a direction to allow anteriorposterior rotation of the upper endplate. Other bearing surface shapeswhich can also be used depending on the type of motion of the mobilecore desired including trough shaped, kidney bean shaped, elliptical, oroval bearing surfaces.

As seen, for example, in FIGS. 2A, 3A, 3C, the superior and inferiorendplates 33 and 35 can each include one or more elongated fins or keels63 and 65, respectively, on the vertebral contacting surface 39 and 41,respectively, thereof. If a single, central fin is provided, there willbe less cutting of bone required during insertion. However, if multiple,usually two, fins, are provided on one or more of the endplates or if afin has a break 63′ between forward and rear components (see FIG. 2A),there can be more bone contact between the fins and increased fixation,among other possible advantages. The fins 63 and 65 as shown have anangled posterior end for ease of insertion and a plurality of angledslots for improved bone fixation.

The superior and inferior endplates 33 and 35 can be configured to bearranged within a disc space to provide motion in the flexion/extensiondirection up to a predetermined angle. In one example, the predeterminedangle is about ±5 to ±15 degrees, and preferably about ±12 degrees ofmotion in the flexion/extension direction, i.e., relative angularmovement of the anterior and posterior ends 33 a and 33 p of thesuperior endplate with respect to the anterior and posterior ends 35 aand 35 p of the inferior endplate. In a presently preferred disc system,for implants placed with the center of the concave bearing surfaces 47,49 of the endplates of the two discs 27 and 29 of the disc system about22 mm apart, the following kinematics can be expected: ±5 degrees ofaxial rotation and ±12 degrees of flexion/extension. The intervertebraldisc system self centers due to the fact that as the core moves awayfrom a neutral centered position, the assembled height of the systemgradually increases. The force of the surrounding tissue tries to bringthe disc back to the lower height configuration of the neutral position.

As seen in FIGS. 3B and 4B, the endplates 33 and 35 can be provided withholes or recesses 91 at an anterior end of the endplates to facilitateremoval of the endplates anteriorly in a subsequent surgical procedureif needed.

The upper vertebral contacting surface 39 of the superior endplate 33(FIGS. 3A-3E) has a central portion 71 that is raised relative to aperipheral portion 73 of the superior endplate. As seen in, e.g., FIG.2B and 3C-3E, the central portion 71 can be raised so that it defines aportion of a sphere, however, as seen in FIGS. 4A-4C, the raised centralportion can have generally spherical (FIG. 6A), stepped (FIG. 6B), orpyramidal or conical (FIG. 6C) shape, as well as other shapes that mightbe determined to be useful. These shapes are described withoutconsidering the shape of the keel 63 which will ordinarily also extendupwardly from the raised central portion 71. The lower bearing surface47 of the superior endplate 33 ordinarily has a concavity disposedopposite the raised central portion 71.

The lower vertebral contacting surface 41 of the inferior endplate 35(FIGS. 4A-4E) has a central portion 75 and the upper bearing surface 49of the inferior endplate also ordinarily has a concavity disposedopposite the central portion. The central portion 75 of the inferiorendplate 35 is ordinarily either not raised or not raised to as great anextent relative to the peripheral portion 77 of the inferior endplate asthe central portion 71 of the superior endplate 33 is raised relative tothe peripheral portion 73 of the superior endplate. Thus, the shape ofthe upper vertebral contacting surface 39 of the superior endplate 33 isordinarily different than the shape of the lower vertebral contactingsurface 41 of the inferior endplate 35. When the central portion 75 ofthe inferior endplate 35 is raised, like the raised central portion 71of the superior endplate 33, it can have generally spherical (FIG. 6A),stepped (FIG. 6B), or pyramidal or conical (FIG. 6C) shape, as well asother shapes that might be determined to be useful.

The height of the superior endplate 33 measured from an outer peripheraledge 79 of the lower bearing surface 47 of the superior endplate to atop 81 of the raised central portion 71 of the upper vertebralcontacting surface 39 is ordinarily greater than a height of theinferior endplate 35 measured from an outer peripheral edge 83 of theupper bearing surface 49 of the inferior endplate to a bottom 85 of thecentral portion 75 of the lower vertebral contacting surface 41. Thesuperior endplate 33 height is ordinarily at least 30% greater than theinferior endplate 35 height. The central portion 75 of the inferiorendplate 35 is ordinarily at least somewhat raised, and the raisedcentral portion 71 of the superior endplate 33 ordinarily has a heightwhich is about 150% to 300% of a height of the central portion of theinferior endplate.

The lower bearing surface 47 and the upper bearing surface 49 ordinarilyeach comprise generally circular outer peripheral edges 79 and 83. Theouter peripheral edge 83 of the upper bearing surface 49 can have alarger diameter D1 (FIG. 4E) than the diameter D2 (FIG. 3E) of the outerperipheral edge 79 of the lower bearing surface 47.

The upper vertebral contacting surface 39 of the superior endplate 33and the lower vertebral contacting surface 41 of the inferior endplate35 may, in addition to the keels 63 and 65, be provided with knurling,teeth, serrations or some other textured surface 67 to increase frictionbetween the vertebral contacting surfaces and the adjacent vertebra. Asseen, for example, in FIGS. 2A and 3A, the textured surface 67 caninclude a plurality of ribs 69 on the upper vertebral contacting surface39 of the superior endplate 33 that can extend onto the raised centralportion 71 of the superior endplate. The raised central portion 71 ofthe superior endplate 33 is raised relative to what is ordinarily agenerally flat, planar peripheral portion 73 of the superior endplate,although the peripheral portion may have a slight slope away from theraised central portion. The teeth and ribs 69 or other structures of thetextured surface extend upwardly from the generally planar peripheralportion 73 of the superior endplate 33. The lower vertebral contactingsurface 41 of the inferior endplate 35 may also have a textured surface,such as at least one of teeth and ribs extending downwardly from thegenerally planar peripheral portion 77 of the inferior endplate.

The upper bearing surface 49 and the lower bearing surface 47 ordinarilyeach comprise part surfaces of spheres (ordinarily but not necessarilyhaving the same radius R1) with generally circular outer peripheraledges 83 and 79. A core retention portion comprises a wall 61 extendingone of downwardly from the outer peripheral edge 79 of the lower bearingsurface 47 (shown in FIG. 3E) and upwardly from the outer peripheraledge 83 of the upper bearing surface 49.

The upper and lower core bearing surfaces 53 and 55 of the core 51ordinarily each comprise generally circular outer peripheral edges 87and 89, respectively, and the core can have a non-spherical transitionregion 91 between the upper and lower core bearing surfaces. The recess59 can be provided in the transition region 91, such as in the form ofan annular groove, and the projection 57 can extend inwardly from thewall 61, such as in the form of an annular protrusion. A height of thetransition region 91 between the upper and lower core bearing surfaces53 and 55 is greater than a height of the wall 61 of the core retentionportion.

By providing the raised central portion 71 on the upper vertebralcontacting surface 39 of the superior endplate 33, the disc 27 can havea reduced height B relative to a disc height C of a conventional disc127 in which both upper and lower vertebral body contacting surfaces ofare flat, as seen by comparing FIGS. 1A and 1B and 2C and 5. Theinventor has recognized that the prior art flat configuration does notgenerally match the surfaces of the vertebral bodies, which results inless than ideal bone to implant contact surfaces. The superior endplate33 with the raised central portion 71 provides a more anatomicallyshaped vertebral body contacting surface for an artificial disc whileproviding the same endplate to endplate spacing A and an equal range ofmotion. As seen in FIG. 1A, by lowering the periphery 73 of the uppervertebral contacting surface 39 of the superior endplate 33 relative tothe periphery 173 of a conventional superior endplate 133, the uppervertebra 200 is allowed to settle into a lower position relative to thelower vertebra. This can, moreover, be accomplished without changing theclearance (ROM) between endplates or the position of the centers ofrotation of the endplates. The raised central portion 71 can, inaddition, facilitate fixation because the raised central portion can sitdeeper into vertebra behind the external cortex, so it is less likely toexpulse or retropulse.

The inventor has also recognized that, when the central portions 71 and75 of both the upper vertebral contacting surface 39 and the lowervertebral contacting surface 41 are raised, it can be advantageous tohave the central portions be raised in different amounts, i.e., domes onboth endplates are not equal because the anatomy of the upper and lowersurfaces of the vertebrae are of different shapes.

FIG. 7 shows the radius of curvature of the superior endplate of thesampling of patients to be between about 10 mm and 30 mm with an averagevalue of about 20 mm. FIG. 7 shows the curvatures of different disclevels with the vertebrae identified by numbers C4-C7 and the vertebralspaces identified by the numbers of the two adjacent vertebrae. FIG. 8shows corresponding numbers for depths of the superior endplatecavities. As a comparison, the inferior endplates were found to besubstantially flat or having such a small curvature that it wasgenerally immeasurable. Based on this data and the interest in designinga smaller height disc, the central portion of the superior endplate 71has been raised by a greater amount than that of the inferior endplateto most closely accommodate the patient anatomy.

The superior and inferior endplates of the two discs may be constructedfrom any suitable metal, alloy or combination of metals or alloys, suchas but not limited to cobalt chrome alloys, titanium (such as grade 5titanium), titanium based alloys, tantalum, nickel titanium alloys,stainless steel, and/or the like. They may also be formed of ceramics,biologically compatible polymers including PEEK, UHMWPE, PLA or fiberreinforced polymers. The endplates may be formed of a one piececonstruction or may be formed of more than one piece, such as differentmaterials coupled together.

The core can be made of low friction materials, such as titanium,titanium nitrides, other titanium based alloys, tantalum, nickeltitanium alloys, stainless steel, cobalt chrome alloys, ceramics, orbiologically compatible polymer materials including PEEK, UHMWPE, PLA orfiber reinforced polymers. High friction coating materials can also beused.

Different materials may be used for different parts of the disc tooptimize imaging characteristics. PEEK endplates may also be coated withtitanium plasma spray or provided with titanium screens for improvedbone integration. Other materials and coatings can also be used such astitanium coated with titanium nitride, aluminum oxide blasting, HA(hydroxylapatite) coating, micro HA coating, and/or bone integrationpromoting coatings. Any other suitable metals or combinations of metalsmay be used as well as ceramic or polymer materials, and combinationsthereof. Any suitable technique may be used to couple materialstogether, such as snap fitting, slip fitting, lamination, interferencefitting, use of adhesives, welding and/or the like.

In the present application, the use of terms such as “including” isopen-ended and is intended to have the same meaning as terms such as“comprising” and not preclude the presence of other structure, material,or acts. Similarly, though the use of terms such as “can” or “may” isintended to be open-ended and to reflect that structure, material, oracts are not necessary, the failure to use such terms is not intended toreflect that structure, material, or acts are essential. To the extentthat structure, material, or acts are presently considered to beessential, they are identified as such.

While this invention has been illustrated and described in accordancewith a preferred embodiment, it is recognized that variations andchanges may be made therein without departing from the invention as setforth in the claims.

What is claimed is:
 1. An intervertebral disc comprising: a superiorendplate having an upper vertebra contacting surface and a lowersurface, wherein the upper surface of the upper endplate has a raiseddome portion and wherein the lower bearing surface has a concavitydisposed opposite the raised dome portion; an inferior endplate having alower vertebra contacting surface and an upper surface, wherein thelower surface of the lower endplate is either not raised or not raisedto as great an extent as the upper surface of the upper endplate; a corepositioned between the superior and inferior endplates, the core havingupper and lower surfaces configured to mate with the superior andinferior endplates.
 2. The disc of claim 1, wherein the upper and lowersurface raised portion heights are measured from the periphery of thevertebra contacting surface, not including any serrations, teeth orfins, to the highest part of the raised dome portion, not including anyserrations, teeth or fins.
 3. The disc of claim 1, wherein a centralportion of the inferior endplate lower vertebra contacting surface israised, and the raised dome portion of the superior endplate has aheight which is about 150% to 300% of a height of the raised centralportion of the inferior endplate.
 4. The disc of claim 1, wherein aplurality of ribs are provided on the upper vertebral contacting surfaceof the superior endplate and extend onto the raised dome portion of thesuperior endplate.
 5. The disc of claim 1, wherein the upper vertebralcontacting surface of the superior endplate has a different shape thanthe lower vertebral contacting surface of the inferior endplate.
 6. Thedisc of claim 1, wherein the raised dome portion is raised relative to agenerally planar peripheral portion of the superior endplate.
 7. Thedisc of claim 1, comprising at least one of teeth and ribs extendingupwardly from the upper vertebral contacting surface of the superiorendplate.
 8. The disc of claim 1, wherein the upper vertebral contactingsurface of the superior endplate comprises a plurality of pyramid shapedteeth on the raised dome portion.
 9. The disc of claim 1, wherein theupper and lower surfaces of the core comprise part surfaces of spheres.10. The disc of claim 9, comprising a non-spherical transition regionbetween the upper and lower surfaces of the core.
 11. The disc of claim1, comprising at least one of teeth and ribs extending downwardly fromthe lower vertebral contacting surface of the inferior endplate.